add missing initializers

contracts/GSN/Context.sol

 pragma solidity ^0.5.0;
 
+import "@openzeppelin/upgrades/contracts/Initializable.sol";
+
 /*
  * @dev Provides information about the current execution context, including the
  * sender of the transaction and its data. While these are generally available
@@ -10,7 +12,7 @@ pragma solidity ^0.5.0;
  *
  * This contract is only required for intermediate, library-like contracts.
  */
-contract Context {
+contract Context is Initializable {
     // Empty internal constructor, to prevent people from mistakenly deploying
     // an instance of this contract, which should be used via inheritance.
     constructor () internal { }

contracts/GSN/GSNRecipientERC20Fee.sol

@@ -33,7 +33,8 @@ contract GSNRecipientERC20Fee is Initializable, GSNRecipient {
      */
     function initialize(string memory name, string memory symbol) public initializer {
         if (_token == __unstable__ERC20PrimaryAdmin(0)) {
-            _token = new __unstable__ERC20PrimaryAdmin(name, symbol, 18, address(this));
+            _token = new __unstable__ERC20PrimaryAdmin();
+            _token.initialize(name, symbol, 18, address(this));
         }
 
         GSNRecipient.initialize();
@@ -116,10 +117,10 @@ contract GSNRecipientERC20Fee is Initializable, GSNRecipient {
  * outside of this context.
  */
 // solhint-disable-next-line contract-name-camelcase
-contract __unstable__ERC20PrimaryAdmin is ERC20, ERC20Detailed, Secondary {
+contract __unstable__ERC20PrimaryAdmin is Initializable, ERC20, ERC20Detailed, Secondary {
     uint256 private constant UINT256_MAX = 2**256 - 1;
 
-    constructor(string memory name, string memory symbol, uint8 decimals, address sender) public {
+    function initialize(string memory name, string memory symbol, uint8 decimals, address sender) public initializer {
         ERC20Detailed.initialize(name, symbol, decimals);
         Secondary.initialize(sender);
     }

contracts/introspection/ERC1820Implementer.sol

 pragma solidity ^0.5.0;
 
+import "@openzeppelin/upgrades/contracts/Initializable.sol";
+
 import "./IERC1820Implementer.sol";
 
 /**
@@ -10,7 +12,7 @@ import "./IERC1820Implementer.sol";
  * {IERC1820Registry-setInterfaceImplementer} should then be called for the
  * registration to be complete.
  */
-contract ERC1820Implementer is IERC1820Implementer {
+contract ERC1820Implementer is Initializable, IERC1820Implementer {
     bytes32 constant private ERC1820_ACCEPT_MAGIC = keccak256(abi.encodePacked("ERC1820_ACCEPT_MAGIC"));
 
     mapping(bytes32 => mapping(address => bool)) private _supportedInterfaces;